1. Field of the Invention
The present invention relates to a band selection method for a dual-band mobile phone having both the functions of a system requiring SIM (subscriber identification module) card, for example, GSM (Global System for Mobile communication) and a system not requiring the SIM card, for example, PHS (personal handy-phone system) and capable of automatically selecting and operating any one of the two systems without inducing an increase in size of the unit structure, and a mobile phone using the same method.
2. Description of the Related Art
Recently, a dual band mobile phone system capable of using different communication systems or different frequency bands with a single mobile phone unit has been developed.
Conventionally, in this kind of band selection method for a dual band mobile phone and its apparatus, user can automatically or manually select a system considered optimum depending on a communication enabled area, busy state of communication lines, communication fee and the like of each system.
For example, a dual band mobile phone having GSM used world wide mainly in Europe and PHS used in mainly Japan and Asia has been developed to a practical use level.
The features of the GSM are use of a frequency band of 900 MHz and a memory card for storing subscriber information, which is called SIM card. The SIM card contains user telephone numbers, accounting information and the like, and if this SIM card is not used, the GSM terminal cannot be used. Although, for the GSM, its cell area is wide because of a large output, it is not capable of communicating underground because no radio network has not been prepared yet and its communication fee is higher than others.
On the other hand, it is the feature of the PHS that frequency band of 1.9 GHz is used, its transmission output power is as small as 10 mW on average, an area which electromagnetic wave can reach is 100 m in radius from a base station, and therefore, this is suitable for a small cell type mobile phone system. Therefore, because the base station can be installed easily at a low cost although the service area is limited to a small area, it allows communication underground if a base station is installed, and further communication fee is cheap.
To solve those problems, a dual band mobile phone suitable for both the systems has been developed.
Conventionally, in this kind of the dual band mobile phone, user must select a system for use. For example, the mobile phone is provided with a mode setting key and by operating this key manually, a system for use is selected. Further, there is a method in which a system for use is automatically selected by incorporating a RSSI (Received Signal Strength Indication) measuring circuit in a common control unit of a mobile phone. In this case, a field intensity level is measured according to a control channel signal received from a base station of each system.
Of the above-described conventional band selection methods for dual band mobile phone, in case where user manually selects a system for use, there is a problem that user""s selection of system is complicated. The reason is that the user must select a system considered optimum depending on the communication enabled area of each system, busy state of the line, communication fee and the like.
In case of automatically selecting a system for use by measuring of the RSSI, there is a problem that the size of the mobile phone is enlarged and production cost is also increased. In any case, when the GSM requiring the SIM card is selected, if the SIM card is not loaded, there occurs such a problem that communication cannot be started while an error message is displayed on a character display.
The present invention has been made to solve the above problems and therefore it is an object of the invention to provide a band selection method for dual band mobile phone which allows automatic selection of system for use without inducing a large increase of production cost and a mobile phone using the same method.
To achieve the above object, the present invention provides a band selection method for dual band mobile phone, having both a first system requiring a SIM card and a second system not requiring the same. This band selection method comprises: detecting whether or not the SIM card is mounted while an operating mode is regarded as a system mode of the first system when a power switch is turned on; and setting the operating mode to a system mode of the first system when the SIM card is detected, while if the SIM card is not detected, setting the operating mode to a system mode of the second system. Therefore, user can use the mobile phone without making consciousness of presence/absence of the SIM card.
In addition to the above, the dual band mobile phone of the present invention includes a mode memory for memorizing a system mode of any one of the two systems. The band selection method comprises: retrieving a system mode memorized in the mode memory when the power switch is turned on, retrieving the SIM card when a system mode of the first system is detected, and setting the operating mode to a system mode of the first system when the SIM card is detected. Further, either when a system mode of the second system is detected as a result of retrieval of the mode memory or when it is detected that the SIM card is not mounted as a result of retrieval of the SIM card, the system mode of the second system is set up. Therefore, because the mobile phone may start the functional operation of the system memorized in the mode memory, the starting operation carried out when the power is turned on can be simplified.
A dual band mobile phone using the above-mentioned method has both a first system requiring a SIM card and a second system not requiring the same. And the dual band mobile phone comprises a card mounting unit for mounting the SIM card, a power switch for supplying a battery power to a predetermined circuit and a common control unit. This common control unit includes an SIM card retrieval circuit and a mode setting circuit. The SIM card retrieval circuit is provided for retrieving the SIM card at the card mounting unit while an operating mode is regarded as a system mode of the first system when the power switch is turned on. And the mode setting circuit is provided for setting the operating mode to a system mode of the first system when the SIM card retrieval circuit detects the SIM card, or setting the operating mode to a system mode of the second system if the SIM card is not detected.
The above dual band mobile phone further comprises a mode memory for memorizing any one system mode of the two systems and a mode detection circuit included in a common control unit. The mode detection circuit is provided for detecting a system mode stored in the mode memory when the power switch is turned on. The above-described SIM card retrieval circuit retrieves the SIM card at the card mounting unit when this mode detection circuit detects a system mode of the first system.
The mode setting circuit of the common control unit may set the system mode of the second system either when the mode detection circuit detects the system mode of the second system or when the SIM card retrieval circuit detects that the SIM card is not mounted.
The aforementioned mode memory is capable of memorizing a system mode operating when the power switch is turned off. Therefore, in continuous calls, a new call is capable of selecting a system of the same condition as the last call, thereby enabling a stabilized communication. Further, it is also possible to provide with a display unit for indicating a system mode set by the common control unit by at least one of characters and a lamp. Therefore, an operating system on use can be recognized.